"Come on, punk! Move faster!"
Frisk panted; their face was damp with sweat, their arms screamed, but they swung themself to the next bar, and then the next. Undyne's yell to move faster seemed absurd- it was all they could do was keep moving at all, panting and watching the end of the bars get closer, forcing themself on.
They were almost there. The relief of reaching the end gave them a spike of energy; and gritting their teeth, they pushed faster, forcing on. Undyne's next words almost caused them to falter- almost.
"Okay, Frisk, when you reach the end, don't just drop off! Turn yourself around and climb up!"

Whimpering, they did so; arms shaking, legs flailing, they managed to drag themself up, before collapsing onto the top of the bars.
"Great work! Don't stop moving, though!" Undyne's energy and enthusiasm was somewhat infectious, and she refused to ever let her kids give up. She was, Frisk thought, a pretty good gym teacher- over the past year, they'd become physically much stronger and faster than they'd ever been, and only some of that was due to playing and climbing in the woods or the good food they now ate. They enjoyed spending the time with Undyne, as well- even if sometimes they cursed her for how hard she pushed them- to the point that, even in the holiday, only days after being up all night watching the stars, here they were, working out with her. They knew their friend enjoyed the excuse to yell someone into finding joy in challenging exercise, too.
They dropped down from the bars. The workout area doubled as a playground behind the school; built primarily from wood and stone, with metal bars painted an emerald green to compliment the natural surroundings. Unlike the front, which was neat and well-trimmed, back here was wild and fun. The kids loved it, and they loved Undyne's classes- right up to the point where they remembered how hard they were.

"Come on, move those feet!" Frisk started doing little jumps; then, at her command, they scrambled up a climbing frame of wood, ropes, and bars. She timed them reaching the top; then them getting back down. She timed them climbing their favourite tree; then had them back on the climbing frame, getting them to touch every rope with their left hand.
Eventually, it was all over. Frisk flopped on the grass, panting, while Undyne wrote down all their times.
"Good work, punk!" She gave them a big, toothy grin. "We'll get you into shape yet!"
"Wh- I'm already in shape!" Frisk forced themself to sit, their arms shaking and their sweaty t-shirt sticking to them.
"Hah! You spend too much time sitting around drawing." After their stargazing, they couldn't argue- they'd been so busy with their inspiration to paint and draw they'd neglected their exercise. This was the first time they'd even left the house since then.
She extended a webbed hand and pulled them to their feet. "You said you wanted to be strong- like me!"
"I'll never be strong like you," Frisk said, smiling back. "But... yeah. Strong is good! As long as I don't end up all... bulky and muscle-y and gross, looking like- urgh- Aaron."
"I don't look like that," Undyne pointed out as they walked into the changing rooms, where Frisk grabbed their towel and a change of clothes and entered a shower cubicle. She leaned against the wall, continuing the conversation through the door as Frisk stripped and got under the cool water.
"You really think you'll end up like that?"
"Well- more, uh, human-shaped." Frisk let the water wash the sweat from their face. Undyne didn't change the subject as they washed, and they continued, their thoughts swirling around their mind. "But I do worry. I mean... What if I look like Dad in five years?"
Undyne snorted. "Somehow I doubt that."
"...Yeah, I know it's not likely." Frisk shut off the water and pulled down their towel, starting to rub themself down. "So, uh, you said you wanted to go to the beach while the school's on break?"

The change of subject worked. They pulled their clothes on while Undyne took over talking; excited about getting to go in the sea, about everyone getting a chance to hang out on the beach together. She almost said something about Alphys in a swimsuit, promptly changing her sentence when Frisk came out of the cubicle, smirking slightly.
"How about you, punk? You're coming, right?!"
"Of course! I mean-" Frisk remembered themself, pausing while they bundled everything into their bag. "I should talk to my mum about it first, but we've mentioned it before, so I'm sure it's fine. I doubt she'd come, but I'll try."
"There's a lot of monsters who want to come- Alphys suggested hiring a minibus and someone to drive it?" Undyne was clearly waiting for Frisk's thoughts on the subject; they widened their eyes and shrugged, before doing their shirt up.
"I guess? I don't know anything about how to do that... But if Alphys and Toriel think it's okay, then we could make a big day of it." Frisk straightened their shirt and swung their bag on their back, scanning the room to make sure they hadn't missed anything. They walked out into the sun together.

The front of the school had the most carefully trimmed hedges and the neatest path in the town. Frisk grinned at a particular sculpture at the front- they had long ago asked Asgore to trim a bush to resemble Papyrus' grin; he had been as good as his word, and the skeleton had been incredibly touched- and excited- by the gesture. Frisk had never seen the need to mention that they were behind it.

The pair of them headed towards home, chatting about the beach, what they would need, and when it was likely to happen. Having settled on a date- two days before most monsters would go back to work or to school- Frisk said goodbye to Undyne, looking forward to some good food and figuring out how to make the beach trip happen with Toriel.
She was sat in a chair outside the house, reading in the sun. She beamed when she saw Frisk approaching, closing her book and placing it on her lap.
"Ah- there you are, my dear! Did you enjoy your morning with Undyne?"
"It was, uh... Vigorous?" Toriel chuckled.
"Well, would you like to help me prepare lunch?"
Frisk agreed and they entered the house together. Frisk left their bag on the table- many of the monsters still preferred to wash clothes by hand, and Toriel was one of them, especially after seeing some of the more adventurous ruin their clothes with machines. They would help her with it later.
For now, they joined her in the kitchen. She had cooked some beans and lentils, and was pulling out some vegetables from the fridge. Frisk pulled some herbs out as well and arranged them on the counter.
Together, they set to work; talking about Frisk's morning workout and the conversation afterwards. Toriel liked the idea of the beach- there was one not too distant from their home, close enough for a day trip, although certainly far enough to require some planning. She was a little reluctant to join them- not wanting to infringe on Frisk's time with their friends, and still being rather wary of being out in human society- but she promised to consider it.

Word spread fast, though. Soon, it wasn't just Frisk's group of friends who wanted to go. The dogs were first; the married couple speaking for the rest of them, excited to visit somewhere new. Greater Dog- or Grey, as Frisk had taken to calling him- came with them, more because he wanted to spend time with Frisk than out of curiosity about the trip. When they left he'd coated their clothes with long white hairs. Not that that was new to Frisk, considering their parents.

When Frisk went to talk to Alphys about the trip she was, thankfully, was one step ahead of them; she had heard the whispers going around and had looked into hiring buses. By this point, there were more requests to join the party than Frisk could handle- and they hid themself in her lounge, while she arranged the transport and sent Undyne to deal with the requests to join them.
Even Toriel had agreed to come; as had Asgore. Alphys had nudged Frisk and giggled at that news- she hadn't given up hope of their reconciling, even a year later. Frisk was becoming more nervous at the size of the group- but more than that, they were excited. A day to see new sights, to sit in the sun and simply enjoy being with their friends. A day to simply be, with no pressures and no expectations.

I think this is the first chapter I've done that focuses primarily on Frisk and Undyne. That feels a bit wrong- but at the same time, I feel like she's less likely to provoke real character development with Frisk, as much as they'd like her company. She's kicked off plans for a trip to the beach, though.

Any ideas on other ways to work in the characters I don't cover much? I'm looking for ways of exploring the different relationships and character development once the next few chapters are done, so if there's anything you'd be interested in seeing me take on, let me know! No promises, but it might inspire a few ideas.

-Ren